THE MESSENGER

No. 35-March 1984

Report on the First ESO-CERN Symposium on "Large Scale Structure of the Universe, Cosmology and Fundamental Physics" G. Setti, ESO

The first ESO-CERN Symposium was held at CERN, deuterium, were produced when the age of the Universe was Geneva, from 21 st to 25th November 1983 and was attended only about 100 seconds, the temperature about one billion by approximately 200 participants. The discussions concen­ degrees and the density of the order of the density of water, in trated on the general field of Cosmology, where the progress a phase that lasted about 8 minutes. At that moment the made in the past twenty years, both in elementary particles Universe was essentially a gaseous mixture composed of and astronomy, has shown that these two fields of basic protons, neutrons, electrons, positrons, neutrinos and anti­ research are merging toward a new and fundamental under­ neutrinos (and perhaps some other exotic particles, such as standing of the laws that govern our Universe. A detailed photinos) immersed in a heat bath of photons. The equilibrium account is contained in the Proceedings of the Symposium between these components is maintained by the weak which will be available in a few months. interaction, one of the four fundamental forces wh ich are The meeting was started with an introductory lecture by believed to govern all natural phenomena.* The weak force D. W. Sciama (Oxford and Trieste) who highlighted the numer­ together with the "hot big-bang" model allows definite predic­ ous and fundamental problems the understanding of which tions about the abundances of primordial elements. As appears to require ajoint effort of particle physicists, astrophy­ pointed out by J. Audouze (Paris), this astrophysics model sicists and cosmologists. limits the number of neutrino types to no more than 4, wh ich is As discussed by L. Woltjer (ESO) ,the astronomical observa­ al ready significantly better than the upper limit of about twenty tions have shown that the Universe is not only expanding, but obtained in particle physics experiments (data on zo decay also strongly evolving in the sense that the physical properties obtained at CERN). Another important constraint that stems of the galaxies have changed with time. For instance, it has from primordial element abundances is that the present been discovered that certain classes of objects, such as radio galaxies and quasars, were more numerous and probably more powerful in the past than they are now. This, together • The four fundamenlai forees are: Ihe electromagnetic force, whleh aels among eleelrieally eharged particles and is transmltled by Ihe pholon (il govems with the discovery of the properties of the so-called universal the slrueture of the atom); the weak force, whieh aels on leplons and hadrons background radiation, has led to the conclusion that the and is transmitled by the bosons W·. W-, Z· (il is responsible for the beta deeay Universe can be described by the most simple homogeneous of radioaelive elemenls); the strang force, whleh aels on hadrons and is and isotropie models of General Relativity, whereby it has Iransmitled by partieles ealled pions and kaons (11 is responsible for Ihe nuelear forees Ihal keep logelher the protons and neutrons in Ihe atomle nuelei; Ihe evolved from a very condensed and very hot phase about 20 slrong inleraelion among the quarks, whieh are supposed to be Ihe eonsliluents billion years ago-the "hot big-bang". This interpretation has of Ihe protons and neutrons. is due to partlcles ealled gluons); and Ihe an additional attraction: it can explain in a very natural way the gravitational force, whieh aets on everything and is Iransmilted by apartieie abundances of certain elements, wh ich would be extremely ealled graviton. [fhe leptons include Ihe eleelron. Ihe muon and Ihe lau partieles and Ihe Ihree difficult to account for by the nuclear processes taking place in eorresponding Iypes of neuirinos. The hadrons include Ihe baryons (whleh stars. Accordingly, the bulk of elements, such as helium and ultimalely deeay inlo proions) and Ihe mesons.] density of baryonic matter in the Universe cannot be more than massive stars with a mass of about one million times the mass about 10% of the "c1osure" density, that is to say, the density of the Sun, has been proposed. To illustrate how unsatisfac­ which divides the model universes derived from general tory the situation is, it suffices to remark that the masses relativity into "open" and "closed". If the density is less than involved in these different proposals extend over a range of at the c10sure density then the Universe is "open" and will least seventy orders of magnitude! expand forever to infinity or, vice-versa, it will reach a max­ A very important theoretical development has taken place in imum size at some time in the future and will then recollapse the past few years with the application of the concepts of under the action of its own gravity. In principle, it should be grand unified theories of physics (GUTs) to cosmology. To possible, by means of astronomical observations, to find out in highlight this let us first briefly summarize some of the basic what kind of universe we live. In practice, however, this entails problems facing the cosmologists. the use of a class of astronomical objects (such as galaxies of Strangely enough, the first problem comes about because a certain type) which should be bright enough to permit a the Universe looks so isotropie. Observations of galaxies and mapping of the Universe in depth and whose intrinsic proper­ extragalactic radio sources (radio galaxies and quasars) ties do not change with the cosmic time. As discussed by A. show that the distribution of condensed matter in space, aside Sandage (Pasadena), despite great efforts it has not yet been from local irregularities, is isotropie to better than one part in a possible to separate the evolutionary effects from those due to hundred. But a much stricter limit is derived from the observa­ the geometry of the Universe. The solution to this fundamental tions of the 3°K universal radiation which, as reported by D. T. problem has probably to await the advent of both the Space Wilkinson (Princeton), appears to be intrinsically isotropie on Telescope and the large telescopes of the future, such as the all angular scales to better than one part in ten thousand. VLT. However, in the framework of the standard "hot big-bang" That most of the matter in the Universe may indeed be in model it can be shown that two hypothetical observers placed, non-Iuminous form has been convincingly argued by S. Faber say, 1800 apart in the sky at a distance corresponding to the (Liek Observatory) on the basis of observations of different last moment in which the universal radiation interacted with types of galaxies, groups and clusters of galaxies. It appears ordinary matter, could not have communicated with each as though in these different types of astronomical conditions other. Technically speaking, this is equivalent to saying that ordinary visible matter makes up only about 10% of the total the "horizons" of the two observers, whose radii increase with mass involved. The nature of this "dark" matter has been the the speed of light (the maximum possible speed), were still weil subject of many speculations and everything, from certain separated. Now the isotropy problem arises because it is types of elementary particles, to mini black-holes, up to very difficult to see how regions of space which had no time from

The ESO guesthouse in Santiago where the visiting astronomers are happy to rest (or one day after more than 20 hours in a plane.

2 the moment of the original explosion to come into physical interaction can look so similar, as indicated by the isotropy Tentative Time-table of Council Sessions measurements mentioned above. The second problem, which in technical terms is known as and Committee Meetings in 1984 the "flatness" problem, is directly related to the density of matter in the Universe. It arises from the simple observation April 13 Seientifie Teehnieal Committee that any deviation of the matter density from the "closure" May 22 Users Committee density increases with cosmic time. Thus, if the present May 23 Finanee Committee density of the Universe is only about 10% of the closure June 4-5 Observing Programmes Committee June 6 Committee of Couneil, Geneva density-as indicated by observations of the content of June 7 Couneil, Geneva baryonic matter-at the time when the Universe was only Oetober 8 Seientifie Teehnieal Committee, Chile about 100 seconds old (or when the compression factor was November 13-14 Finanee Committee - 10 billion), the density of matter deviated from the closure November 27-28 Observing Programmes Committee density only by one part in one hundred thousand. Most November 28 Committee of Couneil researchers consider this fine-tuning very unnatural and, November 29-30 Couneil consequently, believe that the matter density must have All meetings will take plaee at ESO in Garehing unless stated always been very close to the "closure" density. otherwise. The third problem is concerned with the apparent asym­ metry in the matter/anti-matter content of the Universe, that is to say, with the evidence that the Universe is essentially composed of baryons. In the simple standard "hot big-bang" model, there is no reason to think that initially at least the picture, these extremely high energies are reached naturally at Universe was not highly symmetric, with equal numbers of the very beginning of the life history of the Universe, when its baryons and anti-baryons in equilibrium with the radiation age was less than about 10-35 seconds. After this time, the field: baryons and anti-baryons annihilate into photons and, cooling due to the expansion of the Universe brings the vice-versa, photons materialize into baryon/anti-baryon pairs. average energy of the particles below 1014 GeV. Because of the cooling due to the expansion of the Universe, One of the predictions of GUTs is that the proton should eventually all baryon/anti-baryon pairs annihilated giving rise decay with a half-life in the range 1031 -1033 years, very much to a corresponding number of photons that constitute the greater than the age of the Universe (- 2 x 10 10 years). This universal radiation field now observed at a temperature of possibility arises because in the GUTs the quarks, which are 3°K, but somehow a small amount of baryons was left over. the constituents of the baryons, and the leptons are parts of That the deviation from a perfect baryon/anti-baryon sym­ the same picture. As a consequence, the baryonic number metry should have been small is shown by the fact that need not be conserved any more, as had been assumed in the presently there are about 100 million photons per baryon (the classical models of elementary particle theory. The results of a universal radiation has cooled down to its present temperature number of experiments set up to measure the proton half-life of about 3°K because of the expansion of the Universe, but were reviewed by E. Fiorini (Milan). The most stringent result is the number of photons has been conserved). If the Universe now being obtained from the Ohio Morton Salt mine experi­ was baryon symmetric to start with, the above picture of ment wh ich sets a lower limit to the proton half-life of 1.5 x 1032 course implies that the baryon number has not been strictly years. This result already enables one to rule out the simplest conserved (it should be remembered that a baryon and an of the GUT models which predicts that the proton half-life anti-baryon add exactly in the opposite way, giving a total would be at most 1031 years. baryon number identically equal to zero). Because of the non-conservation of the baryonic number, Recent developments in elementary particle physics and one can work out a scheme which leads in the first 10-35 fundamental theory, when applied to cosmology, may in fact seconds to a baryon asymmetric universe, even if one had indicate an elegant way out of these problems. After the started from conditions of perfect symmetry between particles successful confirmation of the Glashow, Salam and Weinberg and anti-particles, in this way explaining one of the basic theory on the unification of electromagnetic and weak forces cosmological problems outlined before. Unfortunately, it is not recently obtained at CERN with the discovery of the W+ and yet possible, within the framework of GUTs, to make a ZO bosons, as discussed by P. Darriulat (CERN), there is now quantitatively precise estimate of the excess of matter over an increased confidence in the theoretical approach to the anti-matter. unification of all fundamental forces. It should be noted that In a related context, G. Giacomelli (Bologna) reviewed the almost a century has elapsed since Maxwell made the first present status of the search for monopoles, the magnetic fundamental step of incorporating electric and magnetic counterparts of the electric charge, whose existence is pre­ forces into one unified scheme--the theory of electromagne­ dicted by the GUTs schemes. Since their mass is enormous tism. As reviewed by P. Fayet (Paris), there are a number of (_1015 GeV), they cannot be produced in the laboratory, but of theoreÜcal models which have been proposed to unify the course might have been produced in the very early phases of electro-weak and strong interactions, generally known as the "hot big-bang" and now pervade the Universe. In fact, the Grand Unified Theories (GUTs). At this moment, it still appears production might have been so copious that one has to invoke difficult to work out definite quantitative predictions, but one "suppression" schemes to avoid conflict with the upper limit quantity wh ich seems to be fairly weil estimated is the energy on their present space density. of the particles above wh ich the models should become exact, Another important feature of the more elaborate GUTs is the that is to say, the unification energy at which the forces lose expectation that neutrinos have non-zero rest mass. Obvi­ their individuality. This energy turns out to be about 1014 GeV, ously, this immediately raises the possibility that "dark" mat­ which corresponds to a mass which is about 1012 times the ter, which may pervade the Universe as previously discussed, mass of the W+ and zo bosons. This means, of course, that a is provided by the neutrino sea. In the standard "hot big-bang" direct verification of the GUTs via the production of the model one can compute, in a fairly accurate way, the number particles which mediate the unified force is unthinkable, density of neutrinos wh ich turns out to be of the same order as except in the too distant future. However, in the "hot big-bang" the photon density in the 3°K universal radiation field, that is

3 to say, about 100 million, or so, per cubic metre. With this kind thermal energy of approximately 1019 GeV above which all four of density a very small mass of neutrinos, corresponding to a fundamental forces of nature are unified, including gravity. The rest-mass energy of a few tens of an electron volt, is already basic physics prevailing at that moment may be correctly sufficient to provide the "closure" density of the Universe. described by the so-called supersymmetric theories, such as However, there appear to be difficulties with this kind of "supergravity". The most remarkable property of these picture. Calculations which simulate the non-linear growth of theories, as reviewed by Fayet, is that they correlate particles structures in the expanding Universe can be compared with with adjacent spins, such as particles with spin 1 and spin 'h, the observed distribution of galaxies which, as discussed in and therefore bring together bosons (such as the photon) and detail by J. H. Gort, appear to cluster on the large scale in fermions (such as the electron and the proton). The existence configurations (superclusters) whose sizes are typically 150 of a number of new particles is then predicted. Thus, in million light-years. In a universe dominated by massive neu­ "supergravity", which originates from supersymmetry by trinos, the characteristic scale on which matter condensations assuming that its properties are locally invariant, one recov­ can form and collapse is c1early controlled by the maximum ers not only the graviton, which is the spin 2 particle wh ich distance neutrinos can travel before they are cooled down, mediates the gravitational field in general relativity, but also a due to the expansion of the universe. As pointed out by J. Silk spin 3/2 particle called the gravitino. Thus, the partner of the (8erkeley), this distance is too large and apparently leads to a photon would be a new spin 1/2 particle called photino. These typical size which by far exceeds the observed c1ustering scale new particles, the "... ino"s, could play an important role in of galaxies. Consequently, if the standard cosmological para­ cosmology and, if massive, they could provide the missing meters hold, one would have to conclude that neutrinos mass needed to close the Universe, avoiding some of the cannot provide the missing mass in the Universe. Clearly, the problems associated with neutrinos as previously i1lustrated. solution to all these problems depends, in the end, on a direct Clearly, supersymmetric theories are still highly speculative measurement of the neutrino mass. According to R. L. Möss• and there is, as yet, no experimental verification of their bauer (Munich), who reviewed the experimental situation for validity. However, it is interesting to note that astronomical the measurements of the masses of the various kinds of observations could shed some light on the existence of the neutrinos, the most recent result of an experiment carried out new particles that are predicted. For instance, Sciama has in the Soviet Union indicates that the rest-mass energy of the pointed out that the annihilation of photinos and anti-photinos, electron neutrino should be at least 20 eV. This result is clearly if they are indeed massive enough, could produce a large of crucial importance and one hopes that other experiments background flux of radiation detectable by far ultraviolet and will soon allow to verify its validity. soft X-ray measurements. An important development in theoretical cosmology, which Thus the Universe appears to provide the "natural" labora­ would in fact provide a solution to the isotropy and flatness tory where one can hope to test fundamental theories of problems discussed earlier, has recently been proposed by physics, while, at the same time, any progress in experimental Guth in the framework of GUTs. After the first 10-35 seconds, particle physics may increase our confidence that these the cooling due to the expansion of the Universe would bring theories can be applied to an understanding of the basic the thermal energy below the grand unification energy of 1014 GeV, the electro-weak and the strong forces would again acquire their identity and one would expect something to happen at the transition time (more technically one expects a SECOND ANNOUNCEMENT decrease in the symmetry properties of the fields). Guth's OF AN ESO WORKSHOP ON basic idea is that essentially nothing happens for a while: the Universe expands and cools down by many factors of ten, THE VIRGO CLUSTER while certain properties of the system remain, so to say, OF GALAXIES "frozen". The system lives for a while in an "excited" state which would drive the expansion in such a way as to enable an Since the first announcement of this workshop was effective exchange of information through the Universe. issued in the previous Messenger (No. 34, December 1983) Essentially, a very small piece of the Universe, which is the date has been fixed and a preliminary programme causally connected in the initial phase of this expansion made. The workshop will be held in Garehing, from where, roughly speaking, the expansion velocity is less than September 4-7, 1984. the velocity of light, is then stretched by a very large factor (of 44 The programme covers the following topics: Redshifts, the order of 10 ), and from this piece our entire Universe is observations in the radio continuum and in H I, infrared made. At the same time, this would lead to a model universe in observations (with special emphasis on IRAS results), which the density is almost exactly the c10sure density. When optical spectroscopy, spiral pattern analysis, galactic con­ the system makes the transition to its "normal" state, the tent and structure, the population of dwarf galaxies, UV and energy which had been "frozen" in the fields is suddenly X-ray observation, the cluster dynamics, and the interac­ released and the Universe is reheated to a thermal energy of tion with the environment. approximately 1014 GeV, and the considerations we outlined Among the invited speakers are: W. Forman, W. K. before apply again. It is as if the Universe was born anew. Huchtmeier, J. Huchra, R. C. Kennicutt, C. Kotanyi, A. These types of theoretical schemes are known as inflationary Sandage, G. A. Tammann and R. 8. Tully. models. As discussed by D. Nanopoulos (CERN), however, Those interested in participating in this workshop and/or there are still a number of difficulties concerned with a fuller presenting contributed papers (probably mostly in the form understanding of the basic physics at work and, in particular, of poster papers) should write to: the most recent investigations lead to models that are affected O.-G. Richter B. Binggelli by inhomogeneities which appear to be so large that their M. Tarenghi Astronomisches Institut presence would contradict what can be allowed in the real European Southern Observatory Venusstr. 7 Universe. However, the basic idea of "inflation" is extremely Karl-Schwarzschild-Straße 2 CH-4102 Binningen appealing and one other possibility is that it can be applied to 0-8046 Garching bei München Switzerland even earlier times, when the Universe was only 10-43 seconds Federal Republic 01 Germany old. This time, known as the Planck time, corresponds to a

4 cosmological problems. Admittedly, we are still at the begin­ most probable state would correspond to an oscillating model ning ofthe road, but the interplay between particle physics and universe, singularity free, which is initially inflating and where cosmology may indeed lead us to a deeper understanding of the entropy does not change with time. the fundamental laws of nature. The Symposium ended with two concluding lectures by A most remarkable paper was presented by S. W. Hawking M.J. Rees (Cambridge) and J. Ellis (CERN) respectively, who (Cambridge) who showed that, under certain plausible summarized beautifully the main items discussed during the assumptions, the Universe may be described by a wave meeting and set the perspective for future work from the function obeying a simple Schrödinger equation such that the astrophysicist's and particle physicist's standpoint.

Chromospheric Emission, Rotation and X-ray Coronae of Late-type Stars R. Pal/avicini, Arcetri Astrophysica/ Observatory, F/orence, /ta/y

In a short note which appeared in 1913 in the Astrophysical These waves, propagating in an atmosphere of rapidly Journal, G. Eberhard and K. Schwarzschild reported on the decreasing density, steepen into shocks and dissipate their observation of emission reversals at the centre of K line of Ca 11 kinetic energy into thermal energy, thus producing the in some bright late-type stars (Arcturus, Aldebaran, a Gem). observed temperature rise. They also noticed that the same phenomenon is usually In its simplest formulation this theory, first suggested by L. observed in active regions on the Sun. To my knowledge, this Biermann and M. Schwarzschild in the late forties and since was the first time that chromospheric emission was reported then universally accepted, neglects the presence of magnetic from stars other than the Sun. Since those early days our fields which are considered as an unnecessary, easily avoid­ knowledge of stellar chromospheres has enormously able, complication. Consequently, generation of acoustic increased, mainly through systematic surveys in the Hand K waves and heating of outer stellar atmospheres are supposed lines of Ca 11. More recently, observations at UV and X-ray to be spatially homogeneous and temporally constant, at wavelengths from space have provided ample evidence that variance with spatially resolved observations ofthe Sun, which chromospheres, transition regions and coronae are common show the chromosphere and corona to be both highly struc­ to stars throughout the HR diagram. What is more significant is tured and time variable. For example, Fig. 1 shows a spec­ that these observations have demonstrated that magnetic troheliogram of the Sun obtained in the K line of Ca 11. fields playafundamental role in the heating of outer stellar Enhanced chromospheric emission is observed from magneti­ atmospheres and that the observed emission levels are in cally disturbed active regions ("plages"), as weil as from the strong qualitative and quantitative disagreement with the predictions of the standard theory of coronal formation via the generation and dissipation of acoustic waves. The emphasis at present is on heating mechanisms wh ich are based on the stressing and dissipation of magnetic fields generated by dynamo action in subphotospheric convection zones. As a result of this, stellar rotation has come to playa central role in the heating problem, as a controlling factor of the efficiency of the dynamo process. It can be anticipated that in the near future new accurate determinations of stellar rotation rates, as weil as new measurements of transition regions and coronal emission from space will substantially increase our under­ standing of the process of coronal magnetic heating in late­ type stars.

Chromospheric and Coronal Heating In the solar atmosphere, the temperature, after decreasing outwards to a minimum value of = 4,500 K in the upper photosphere, starts to rise again, reaching 104 K in the chromosphere and more than one million degrees in the corona. Since heat cannot flow from lower to higher tempera­ ture regions (second law of thermodynamics) the observed temperature rise requires a non-thermal energy flux to be added to the thermal flux generated by thermonuclear reac­ tions in the core of the Sun and flowing outwards under the form of radiation and convection. For stars of spectral type w~ich later than early F-stars are known on theoretical Fig. 1: Spectrohefiogram ofthe Sun in the K fine ofCa 1/ obtainedat the grounds to possess outer convection zones-the required Solar Tower ofthe Arcetri Observatory. Notice the enhanced emission energy flux has been traditionally ascribed to the generation of from the chromospheric network and from magnetical/y disturbed acoustic waves by turbulent motions in the convection zone. active regions.

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